Japanese Patent Publication No. 2004-278371 (JP2004-278371A) (Patent Document 1) discloses, in FIGS. 1 to 4, a counter-rotating axial-flow fan including a housing constituted by a housing body having an air channel therein, which has a suction port portion on one end thereof and a discharge port on the other end in an axial direction and a motor support frame arranged in a center portion of the air channel. In this counter-rotating axial-flow fan, a first impeller rotated by a first motor is arranged in a first space between the motor support frame in the housing and suction port. Further, a second impeller rotated by a second motor is arranged in a second space between the motor support frame in the housing and discharge port. The first and second impellers are rotated in mutually opposite directions. The first and second motors each have a rotary shaft, a stator, and a rotor rotating around the stator. The first and second motors are arranged respectively in the first and second spaces such that the axial lines of the rotary shafts thereof extend along a virtual center line connecting the center of the suction port and the center of the discharge port. The stator of the first motor and stator of the second motor are fixed to the support frame body. A problem with a motor of this kind is to suppress an increase in the temperature of the inside of the motor. Japanese Utility Model Publication No. 58-159889 (JPU 58-159889A) (Patent Document 2) and Japanese Patent Publication No. 10-210727 (JP 10-210727A) (Patent Document 3) disclose a technique by which to cool a stator of a single motor by using air introduced through a vent hole formed in a rotor of the motor. Further, Japanese Patent Publication No. 2000-356200 (JP2000-356200A) (Patent Document 4) discloses a technique by which to circulate air introduced into an interior of a motor through a vent hole formed in a casing of the motor by means of rotations of an impeller of the motor in a single axial-flow fan.